Many communication systems are based on wireless transmissions. Transceivers of these systems transmit and receive certain information and couple to antennas to convert electrical currents into electromagnetic waves and vice versa. A transceiver's performance with respect to power consumption and transmitter linearity is typically determined by the performance of a radio frequency power amplifier.
A radio frequency amplifier is a circuit to amplify a radio frequency signal such that it can be transmitted wirelessly via an antenna. The amplifier can have only a single amplifying stage or can have multiple amplifying stages.
The radio frequency signal is typically generated by a modulator which modulates a high frequency carrier signal by a baseband signal. The baseband signal represents the information to be transmitted in its non-shifted spectrum. The properties of the carrier signal to be modulated may be amplitude, a frequency and phase or a combination of these.
A high linearity of amplification of a radio frequency signal is important when a radio frequency signal with a modulation of amplitude and phase with a radio frequency carrier signal is amplified. Examples of such signals include signals resulting from data modulation techniques like QAM, QPSK and OFDM. In case of a substantial non-linear operation of the amplifier the output signal spectrum may contain unwanted frequencies and therefore cause other radio channel distortions. It is therefore highly desirable to operate the amplifier circuit according to predetermined linearity requirements.
In addition to linearity particularly for handheld applications like cell phones, PDA's or other wireless mobile devices, the power consumption of the radio frequency amplifier is another performance criterion. Lower power consumption leads directly to longer battery lifetime and is therefore a contribution for saving valuable environmental resources. The operating point (working point) of a radio frequency amplifier is typically set in a way that the amplifier operates clearly in the linear area of its current-voltage characteristic. However, in this case the power consumption of the amplifier is normally quite high.